The present invention relates generally to sensing circuits and more particularly to improved circuitry for sensing and indicating the conductive state of a semiconductor device.
In many applications of semiconductor devices it is highly desirable to be able to determine the conductive state of the device at all times. For example, when thyristors are used in power conversion systems, knowledge of the conductive state of each of the thyristors at each instant in time is necessary in order to know whether the system is working properly. A thyristor, as is well known, belongs to that group of semiconductor devices which is rendered conductive by the simultaneous application of a forward bias voltage between its anode and cathode and a gating signal applied to a third (gating) electrode. A thyristor is rendered nonconductive (i.e., commutated) by the application of a zero or reverse bias voltage across its anode and cathode for a period of time sufficient to allow conduction to cease.
In the typical thyristor power conversion system, a plurality of thyristors are connected in a bridge arrangement and conversion from alternating current (a.c.) to direct current (d.c.) or d.c. to a.c. is achieved through the proper sequencing of the conductive state of the several thyristors. A common problem with such conversion systems is that, for any of a number of reasons, one or more thyristors of the conversion bridge will either fail to conduct or to commutate at the proper time. Failure to commutate is usually a more serious problem since, if not immediately corrected, one commutation failure or commutation fault often leads to additional commutation failures and soon control of this system is lost and damage to the equipment or injury to personnel could result.
A wide variety of schemes are presently employed to detect the conductive condition of semiconductor devices and hence to determine a potential or existing fault condition. Some of these schemes are fairly elaborate and while providing very good results are also relatively expensive. One of the simpler and more widely used schemes in an a.c. to d.c. conversion system is to sense the voltage across the semiconductor device as an indication of its conductive state to thus determine the existence of a commutation fault. While this basic scheme has the advantage of low cost because sensing of a voltage is a relatively easy and inexpensive procedure, the actual implementation of this basic idea becomes more complex since, although the voltage across the semiconductor device will approach the zero level when the device is conductive, it will also go to zero volts each time the a.c. source voltage goes through zero. Thus, in order to properly implement this scheme, allowances must be made to account for the zero source voltage crossings and to distinguish these zero voltage conditions from those which exist when the semiconductor device is conducting. One way of achieving this is to maintain accurate timing with respect to the a.c. source and not sample the device voltage when the a.c. source is going through zero. A more common method is to take repeated samples of the voltage across the device so that only a substantially zero voltage indication at consecutive sampling intervals will actually be recognized as a conducting semiconductor. These solutions to the basic problem obviously result in a more complex and more costly scheme and tend to reduce the overall reliability of the detection scheme.